someone who is either 20 or in their early twenties but might as well be considered a teenager due to their immaturity and/or lack of life skills/experiences.

Heather: Joe asked me out!
Michelle: that guy from best buy? how old is he?
Heather: 20...only 7 yrs younger..
Michelle: eww he is twentyteen! you might as well add a teen to that age he is so young..unless he is sexy dont bother!